Americans More Likely to Feel Down, Stressed, on Hot Days

By Linda Carroll

March 27, 2020

(Reuters Health) - Hot days increase the probability that the average American adult will report bad mental health, a new study suggests.

Based on 17 years of U.S. survey data, researchers found participants reported better mental health when temperatures were cooler than 60 to 70 degrees Fahrenheit (about 15-21 degrees Celsius) and worse mental health when it was hotter, according to the report in PLoS ONE.

The effect was especially pronounced in people with preexisting mental health issues, said Mengyao Li, an applied economist at the University of Georgia, in Athens, who led the study.

"These people could get worse on hot days," Li said. "And doctors seeing them should be prepared for that."

Li also expects more people to experience bad mental health days if temperatures overall continue to rise with climate change.

Li and her colleagues used data from the Behavioral Risk Factor Surveillance System (BRFSS), an ongoing state-based system of health surveys conducted each year under the auspices of the Centers for Disease Control and Prevention, along with county-level temperature data.

The BRFSS is given every month to a different set of people in the U.S. and collects demographic information on age, sex, marital status, number of children, education, employment, annual household income, and whether the respondents have healthcare coverage.

Starting in 1993, participants were asked: "Thinking about your mental health, which includes stress, depression and problems with emotions, how many days during the last 30 days was your mental health not good?" Possible answers range from 0 to 30 days.

Based on survey responses through 2010, Li's team found that each additional day above 80 degrees F was associated with a 0.3% increase in the probability of self-reported mental health difficulties for the past month. Each additional day with an average temperature below 20 degrees F (-6.66 C) was tied to a 0.8% reduction in the probability of self-reported mental health difficulties for the past month.

Colder temperatures had an immediate effect, the researchers note, "whereas hotter days tend to matter most after about 10 days."

"The take-home is an important one in the context of ongoing climate change which tends to take things in the warmer direction in most places," said Dr. Paul Nestadt, an assistant professor in the department of psychiatry and behavioral sciences at the Johns Hopkins School of Medicine and the Johns Hopkins Bloomberg School of Public Health in Baltimore. "It's important to recognize that extended periods of very hot days can have an impact on people's stress levels."

One possible reason is that hot weather can impact people's ability to sleep, whereas cold weather does not, Nestadt said.

Still, he noted, the study has a big limitation. "Mental health is a technical term meaning the absence of mental illness," Nestadt said. "What they are saying is that people are reporting more bad days when it's hot. You could argue that these are more stressed out days, but you can't say depression goes up when it gets hot."

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